Hi,

To do that, please go to Customize > General Options > make sure you don't check to "Enable Body Boxed".

After that, try add more this code at the end of functions.php file like on this image:

/**
 * Enable body boxed layout for homepage only
 */
if( ! function_exists( 'penci_enable_body_boxed_homepage_only' ) ) {
    function penci_enable_body_boxed_homepage_only( $data ) { 
 if( is_home() || is_front_page() ) {
 return true;
 } else {
 return $data;
 }
    }
    add_filter( 'theme_mod_penci_body_boxed_layout', 'penci_enable_body_boxed_homepage_only' );
}

Best Regards,

PenciDesign